1. Field of the Invention
The present invention relates to the preparation, via flash pyrolysis, of ceramic powders based on metal and/or metalloid nitrides and/or carbides.
The present invention also relates to the amorphous or crystalline powders thus produced, having particle sizes in the nanometer range.
2. Description of the Prior Art
Processes for the production of silicon carbonitride powder are known to this art. Among these processes, exemplary thereof is a vapor-phase process (U.S. Pat. No. 4,036,653) employing tetrachlorosilane, ammonia and methane starting materials. The proportion of the silicon carbide and silicon nitride phases present in the final powder is determined by the composition of the initial gaseous mixture. This process presents the disadvantage of producing hydrochloric acid and ammonium chloride. The final silicon carbonitride powder has a specific surface higher than 1 cm.sup.2 /g and contains 0.1% to 5% of carbon.
Gonsalves et al, Advanced Materials, 3, No. 4, pp. 202-204 (1991), describe a process which includes forming an aerosol by spraying fine droplets of a precursor material within a laser beam. The precursor employed is a mixture of monocyclic organosilazanes containing 6 and 8 atoms in the ring moiety and including a few linear oligomers. A diffuser equipped with ultrasound-generating means must be employed for forming the drops. The amorphous powder grains thus obtained have a mean diameter of 62 nm. The crystalline powder particles have a bimodal distribution in which the mean diameter values are 44.8 and 119.43 nm.
U.S. Pat. No. 4,594,330 describes a vapor-phase process which entails heating an organosilicon compound devoid of halogen and oxygen atoms. According to this process, the compound must be capable of being vaporized. This process makes it possible to produce an amorphous powder which has grains smaller than 0.2 .mu.m in size, or a crystalline powder.
Mizutani and Liu, Ceramic Powder Science III, p. 59 (1990), describe the pyrolysis of polysilazanes in a conventional furnace. The process includes a stage of formation of fine droplets which have a diameter of a few micrometers, and this mandates that the polysilazane be employed in dilute solution. The amorphous powder thus obtained is in the form of particles which have a mean diameter of 0.38 .mu.m, exhibiting a geometric standard deviation of 1.5.